Thanks for your feedback, Paul! My approach is basically the same as what you had planned: First, decompile levels.dat back to some readable ASCII files. Then, write a level editor that works with these ASCII files. So far, I've been able to locate the individual levels in levels.dat. Inside each level, I can see the hexagonal map (more details below). I can even change individual bytes in levels.dat and get different tiles when restarting hex-a-hop. What I'm still struggling with are the bytes before...
Level editor for Hex-a-hop